[Hallicrafters] HT-37 power supply

[Glen Zook]

The problem with the HT-37 (and the HT-32 series as
well) is that the power transformer is operating on
the "ragged edge" with the tube type rectifiers. 
Transformer failure is fairly common with those units.
 By removing the tube rectifiers from the circuit and
removing the 5 VAC filament connections you eliminate
the load from these filament windings which definitely
helps with the life of the power transformer.

Also, by removing the 5 VAC windings from the
rectifier "string" you also eliminate the possibility
of those windings shorting because of the high voltage
DC which has to pass through them.

You can drop the B+ and B++ (along with things like
the heater/filament voltages) by using a "bucking"
transformer in the AC input line.  Remember that the
vast majority of major electric companies (and many of
the "minor" ones as well) now run 125 volts +/- 2
volts with things on the "+" side being VERY common
especially during warmer weather (to provide for air
conditioning).  This higher input voltage contributes
to the higher voltage as well as going to solid-state.
 By dropping the input voltage the high voltage will
probably drop enough that you won't have any problems.

Glen, K9STH


--- Robert Nickels <w9ran at oneradio.net> wrote:

> Thinking about going to solid state rectifiers in
> the power supply of an 
> HT-37.  I know Al Parker and others have recommended
> this as a way of 
> minimizing the potential for transformer shorts from
> HV to 5 volt 
> windings, but I'm also concerned about the increase
> in voltage that will 
> result.    In fact, I did try the LV supply and
> ended up with over 360 
> volts on the low B+ line, which messed up the relay
> keying tube 
> operation.   Putting suitable dropping resistors in
> series would 
> probably generate more heat than the tubes do!
> 
> I'm tempted to leave it as-is, with the 5R4/5U4
> tubes, but would 
> appreciate others experiences.









Glen, K9STH

Website:  http://k9sth.com


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